Zoom lens system

ABSTRACT

The present invention provides a compact zoom lens system suitable for a video camera and a digital still camera using a solid-state imaging device, having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60°, and superb optical performance. According to one aspect, in order from an object side, a first lens group has negative refractive power, a second lens group has positive refractive power, and a third lens group has positive refractive power. The front principal point of the second lens group is located to the object side of the most object side surface of the second lens group. The distance between the first and second lens group is decreased and the distance between the second and third lens group is increased upon zooming from a wide-angle end state to a telephoto end state. Predetermined conditional expressions are satisfied.

[0001] This application claims the benefit of Japanese Patent application No. 2001-065242 which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a zoom lens system suitable for a video camera and a digital still camera using a solid-state imaging device and, in particular, relates to a compact zoom lens system having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60°, and superb optical performance.

[0004] 2. Related Background Art

[0005] So far zoom lens systems disclosed in Japanese Patent Application Laid-Open No. 11-52246 have been known to be those suitable for a solid-state imaging device.

[0006] However, zoom lens systems disclosed in Japanese Patent Application Laid-Open No. 11-52246 have relatively small zoom ratio of about three. Therefore, it has been a problem to be insufficient zoom ratio.

SUMMARY OF THE INVENTION

[0007] The present invention is made in view of the aforementioned problem and has an object to provide a compact zoom lens system, which is suitable for a video camera and a digital still camera using a solid-state imaging device, having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60°, and superb optical performance.

[0008] According to one aspect of the present invention, a zoom lens system includes, in order from an object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group having positive refractive power. The front principal point of the second lens group is located to the object side of the most object side surface of the second lens group. The distance between the first lens group and the second lens group is decreased and the distance between the second lens group and the third lens group is increased, when the state of lens group positions is changed from a wide-angle end state to a telephoto end state. The following conditional expressions (1), (2), and (3) are satisfied:

0<H1/fw<0.6  (1)

1.0<H2/fw<2.2  (2)

1.6<f3/f2<3.0  (3)

[0009] where H1 denotes the distance between the front principal point of the second lens group and the most object side surface of the second lens group, H2 denotes the distance between the rear principal point of the second lens group and the most image side surface of the second lens group, fw denotes the focal length of the zoom lens system in the wide-angle end state, f2 denotes the focal length of the second lens group, and f3 denotes the focal length of the third lens group.

[0010] In one preferred embodiment of the present invention, the first lens group and the second lens group are moved and the third lens group is fixed when the state of lens group positions is changed from the wide-angle end state to the telephoto end state. The following conditional expression (4) is satisfied:

2.6<M/fw<3.0  (4)

[0011] where M denotes the moving distance of the second lens group along the optical axis between the position locating the most object side and that locating the most image side upon zooming.

[0012] In one preferred embodiment of the present invention, the second lens group is composed of, in order from the object side, a first lens unit having positive refractive power, a second lens unit having negative refractive power, a third lens unit having negative refractive power, and a fourth lens unit having positive refractive power.

[0013] The following conditional expressions (5), (6), (7), and (8) are satisfied:

0.6<f21/f2<1.0  (5)

0.7<|f22|/f2<2.0 (f22<0)  (6)

2.0<f23|/f2<10.0 (f23<0)  (7)

1.2<f24/f2<3.0  (8)

[0014] where f21 denotes the focal length of the first lens unit, f22 denotes the focal length of the second lens unit, f23 denotes the focal length of the third lens unit, and f24 denotes the focal length of the fourth lens unit.

[0015] In one preferred embodiment of the present invention, the first lens unit includes a cemented lens constructed by, in order from the object side, a double convex positive lens having an aspherical surface facing to the object side cemented with a negative meniscus lens.

[0016] In one preferred embodiment of the present invention, the second lens unit includes a cemented lens constructed by, in order from the object side, a double convex positive lens cemented with a double concave negative lens.

[0017] In one preferred embodiment of the present invention, the third lens unit includes a negative meniscus lens having a convex surface facing to the object side.

[0018] In one preferred embodiment of the present invention, the fourth lens unit includes a positive meniscus lens having a convex surface facing to the object side.

[0019] In one preferred embodiment of the present invention, the first lens group is composed of, in order from the object side, a negative meniscus lens having a convex surface facing to the object side, a double concave negative lens having a stronger concave surface facing to the image side, and a positive meniscus lens having a convex surface facing to the object side.

[0020] In one preferred embodiment of the present invention, the third lens group is composed of a double convex positive lens having at least one aspherical surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a drawing showing a lens configuration of a zoom lens system according to Example 1 of the present invention.

[0022]FIG. 2 is a drawing showing a lens configuration of a zoom lens system according to Example 2 of the present invention.

[0023] FIGS. 3A-3E are graphs showing various aberrations in the wide-angle end state according to Example 1 of the present invention.

[0024] FIGS. 4A-4E are graphs showing various aberrations in the intermediate focal length state according to Example 1 of the present invention.

[0025] FIGS. 5A-5E are graphs showing various aberrations in the telephoto end state according to Example 1 of the present invention.

[0026] FIGS. 6A-6E are graphs showing various aberrations in the wide-angle end state according to Example 2 of the present invention.

[0027] FIGS. 7A-7E are graphs showing various aberrations in the intermediate focal length state according to Example 2 of the present invention.

[0028] FIGS. 8A-8E are graphs showing various aberrations in the telephoto end state according to Example 2 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] Zoom lens systems according to Examples of the present invention are going to be explained in accordance with accompanying drawings. FIGS. 1 and 2 are drawings showing lens configurations of zoom lens systems according to Example 1 and Example 2 of the present invention, respectively.

[0030] The zoom lens system according to each Example is composed of, in order from an object side, a first lens group G1 having negative refractive power, an aperture stop AS, a second lens group G2 having positive refractive power, and a third lens group G3 having positive refractive power.

[0031] The first lens group G1 is composed of, in order from the object side, a lens unit U11 constructed by a negative meniscus lens having a convex surface facing to the object side, a lens unit U12 constructed by a double concave lens having a stronger concave surface facing to an image side than that facing to the object side, and a lens unit U13 constructed by a positive meniscus lens having a convex surface facing to the object side.

[0032] The second lens group G2 is composed of, in order from the object side, a first lens unit U21 having positive refractive power, a second lens unit U22 having negative refractive power, a third lens unit U23 having negative refractive power, and a fourth lens unit U24 having positive refractive power.

[0033] The first lens unit U21 is composed of a cemented lens constructed by, in order from the object side, a double convex positive lens having an aspherical surface facing to the object side cemented with a negative meniscus lens. The second lens unit U22 is composed of a cemented lens constructed by, in order from the object side, a double convex positive lens cemented with a double concave negative lens. The third lens unit U23 is composed of a negative meniscus lens having a convex surface facing to the object side. The fourth lens unit U24 is composed of a positive meniscus lens having a convex surface facing to the object side.

[0034] The third lens group G3 is composed of a lens unit U31 constructed by a double convex positive lens having an aspherical surface facing to the image side.

[0035] With above described construction, when the state of lens group positions is changed from a wide-angle end state (a state providing the shortest focal length) to a telephoto end state (a state providing the longest focal length), the first lens group G1 and the second lens group G2 move while the third lens group G3 does not move. Thus, the distance between the first lens group G1 and the second lens group G2 is decreased and the distance between the second lens group G2 and the third lens group G3 is increased. The following conditional expressions (1) through (8) should be satisfied:

0<H1/fw<0.6  (1)

1.0<H2/fw<2.2  (2)

1.6<f3/f2<3.0  (3)

2.6<M/fw<3.0  (4)

0.6<f21/f2<1.0  (5)

0.7<|f22|/f2<2.0 (f22<0)  (6)

2.0<|f23|/f2<10.0 (f23<0)  (7)

1.2<f24/f2<3.0  (8)

[0036] where H1 denotes the distance between the front principal point of the second lens group and the most object side surface of the second lens group, H2 denotes the distance between the rear principal point of the second lens group and the most image side surface of the second lens group, fw denotes the focal length of the zoom lens system in the wide-angle end state, f2 denotes the focal length of the second lens group, f3 denotes the focal length of the third lens group, M denotes the moving distance of the second lens group along the optical axis between the state locating the most object side and that locating the most image side upon zooming, f21 denotes the focal length of the first lens unit, f22 denotes the focal length of the second lens unit, f23 denotes the focal length of the third lens unit, f24 denotes the focal length of the fourth lens unit.

[0037] Conditional expression (1) is to realize a high zoom ratio. When the ratio H1/fw falls below the lower limit of conditional expression (1), it becomes difficult to sufficiently narrow the distance between the rear principal point of the first lens group and the front principal point of the second lens group in the telephoto end state. Accordingly, it becomes difficult to realize a high zoom ratio. On the other hand, when the ratio exceeds the upper limit of conditional expression (1), the power distribution in the second lens group becomes excessively telephoto state. Accordingly, it becomes difficult to realize good correction of aberration.

[0038] Conditional expression (2) is to realize good correction of aberration. When the ratio H2/fw falls below the lower limit of conditional expression (2), the construction of the second lens group becomes relatively simple, so that the degree of freedom to correct aberrations becomes small. Accordingly, in order to realize a high zoom ratio, correction of various aberrations becomes difficult. On the other hand, when the ratio exceeds the upper limit of conditional expression (2), the total lens length of the second lens group becomes large. Accordingly, the zoom lens system becomes large.

[0039] Conditional expression (3) is to realize compactness of the zoom lens system and to obtain an appropriate position of the exit pupil. By the way, it is desirable for a zoom lens system used for a solid-state imaging device that the position of the exit pupil locates sufficiently away in the object direction from the rear principal point of the zoom lens system. When the ratio f3/f2 falls below the lower limit of conditional expression (3), the refractive power of the second lens group becomes small. Accordingly, when a high zoom ratio is to be realized, the total length of the zoom lens system becomes large. On the other hand, when the ratio exceeds the upper limit of conditional expression (3), the position of the exit pupil tends to be close to the image plane in the wide-angle end state, so that it is not desirable.

[0040] Conditional expression (4) is for realizing both high optical performance and compactness with keeping a high zoom ratio. When the ratio M/fw falls below the lower limit of conditional expression (4), the production of various aberration increases. Accordingly, it becomes difficult to realize high optical performance. On the other hand, when the ratio exceeds the upper limit of conditional expression (4), the zoom lens system becomes large, so that it is not desirable.

[0041] Conditional expressions (5), (6), (7), and (8) are for realizing good correction of aberration of the second lens group. When these ratios exceed or fall blow the upper limit or the lower limit of these conditional expressions, respectively, it becomes difficult to realize good aberration correction with keeping both high zoom ratio and compactness of the zoom lens system.

[0042] Various values associated with Examples 1 and 2 are listed in Tables 1 and 2, respectively. In each specification, f denotes the focal length, B. f. denotes the back focal length, FNO denotes the f-number, and 2A denotes an angle of view.

[0043] In lens data, the first column is a surface number counted in order from the object side, the second column “r” is a radius of curvature of a lens surface, the third column “d” is a distance between adjacent lens surfaces, the fourth column “v” is Abbe number, and the fifth column “n” is refractive index for d-line (λ=587.6 nm) where refractive index of air (n=1.00000) is abbreviated.

[0044] In aspherical data, an aspherical surface coefficient is expressed by the following equation:

x(y)=y ² /[r·{1+(1−K·y ² /r ²)^(½) }]+C4·y ⁴ +C6·y ⁶ +C8·y ⁸ +C10·y ¹⁰

[0045] where X(y) denotes the distance along the optical axis from the tangent plane on the vertex of the aspherical surface to the position of the aspherical surface at the height of y, r denotes a paraxial radius of curvature, K denotes the conical coefficient, and Ci denotes i-th order aspherical surface coefficient.

[0046] In zooming data, values of the focal length and varying distances in the wide-angle end state, intermediate focal length state, and telephoto end state are respectively shown.

[0047] In the tables for various values, “mm” is generally used for the unit of length such as the focal length, a radius of curvature, a distance between the adjacent surfaces. However, since an optical system proportionally enlarged or reduced its dimension can be obtained similar optical performance, the unit is not necessary to be limited to “mm” and any other suitable unit can be used.

[0048] Values for conditional expressions according to each Example are listed in Table 3. TABLE 1 [Specification] Wide-Angle Intermediate Telephoto f = 1.000 1.940 3.762 B.f. = 0.350 FNO = 2.64 3.55 5.34 2A = 62.54° 32.48° 16.68° [LensData] r d ν n 1 3.0112 0.1578 46.53 1.80400 2 1.1780 0.4126 3 −6.1499 0.1335 46.58 1.80400 4 3.1539 0.0485 5 2.0520 0.2791 23.78 1.84666 6 11.3500 (d 6) 7 ∞ 0.2184 Aperture Stop 8 1.3621 0.4612 55.18 1.66547 Aspherical Surface 9 −1.3047 0.1092 37.17 1.83400 10 −2.9325 0.0243 11 1.1780 0.2913 58.54 1.65160 12 −11.4357 0.1456 34.96 1.80100 13 0.7642 0.0850 14 1.7353 0.2913 34.96 1.80100 15 1.1012 0.4248 16 1.4143 0.3034 41.49 1.57501 17 4.8747 (d17) 18 15.7767 0.2427 57.44 1.60602 19 3.3516 0.0485 Aspherical Surface 20 ∞ 0.3641 64.10 1.51680 21 ∞ [Aspherical Data] Surface Number: 8 K = 1.00000 C4 = −0.0479300 C6 = −0.0241273 C8 = −0.0483277 C10 = −0.0661716 Surface Number: 19 K = −57.8817 C4 = −0.0860311 C6 = 0.278699 C8 = −0.251478 C10 = 0.000000 [Zooming Data] Wide-Angle end Intermediate Telephoto end f 1.00000 1.94053 3.76213 d 6 3.07158 1.22258 0.27070 d17 0.32105 1.27394 3.11948

[0049] TABLE 2 [Specification] Wide-Angle Intermediate Telephoto f = 1.000 1.942 3.762 B.f. = 0.242 FNO = 2.55 3.44 5.17 2A = 62.54° 32.36° 16.68° [LensData] r d ν n 1 3.1105 0.1578 46.58 1.80400 2 1.1962 0.4368 3 −5.2567 0.1335 59.47 1.53996 4 3.0142 0.0412 5 1.8860 0.2762 23.78 1.84666 6 4.4021 (d 6) 7 ∞ 0.2184 Aperture Stop 8 1.4413 0.3922 49.32 1.74330 Aspherical Surface 9 −2.0224 0.1092 25.43 1.80518 10 −4.5470 0.0243 11 1.4603 0.3053 60.69 1.56384 12 −4.9732 0.2849 34.96 1.80100 13 0.8323 0.0754 14 1.3632 0.1823 34.96 1.80100 15 1.1079 0.4478 16 1.6548 0.2256 47.93 1.71700 17 4.3291 (d17) 18 8.5232 0.2427 59.38 1.58313 19 −2.8160 0.1214 Aspherical Surface 20 ∞ 0.4000 64.14 1.51633 21 ∞ [Aspherical Data] Surface Number: 8 K = 1.00000 C4 = −0.0430070 C6 = −0.0114119 C8 = −0.0187353 C10 = 0.0245463 Surface Number: 19 K = 1.00000 C4 = 0.120353 C6 = −0.162829 C8 = 0.458834 C10 = −0.558854 [Zooming Data] Wide-Angle end Intermediate Telephoto end f 1.00000 1.94175 3.76212 d 6 3.09912 1.22825 0.26699 d17 0.35434 1.31913 3.18403

[0050] TABLE 3 [Values for Conditional Expressions] Example 1 Example 2 (1) H1/fw 0.405 0.445 (2) H2/fw 1.755 1.730 (3) f3/f2 2.331 1.840 (4) M/fw 2.798 2.830 (5) f21/f2 0.825 0.786 (6) |f22|/f2 1.418 1.015 (7) |f23|/f2 2.405 5.445 (8) f24/f2 1.708 1.815

[0051] FIGS. 3A-3E through 5A-5E are graphs showing various aberrations in the wide-angle end state, intermediate focal length state, and telephoto end state according to Example 1 of the present invention, respectively. FIGS. 6A-6E through 8A-8E are graphs showing various aberrations in the wide-angle end state, intermediate focal length state, and telephoto end state according to Example 2 of the present invention, respectively.

[0052] In graphs for various aberrations in each figure, FNO denotes the f-number, d denotes d-line (λ=587.6 nm), and g denotes g-line (λ=435.6 nm). A denotes a half angle of view. In the diagrams showing spherical aberration, astigmatism, and distortion, A denotes the maximum value of a half angle of view. In the diagrams showing coma, A denotes a half angle of view for each image. In the diagrams showing astigmatism, a solid line indicates a sagittal image plane and a broken line indicates a meridional image plane.

[0053] As is apparent from the respective graphs, the zoom lens system according to each Example shows superb optical performance as a result of good corrections to various aberrations.

[0054] As described above, the present invention makes it possible to realize a compact zoom lens system, which is suitable for a video camera and a digital still camera using a solid-state imaging device, having a zoom ratio of 3.5 or more, an angle of view in a wide-angle end state of about 60°, and superb optical performance.

[0055] Additional advantages and modification will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 

What is claimed is:
 1. A zoom lens system comprising, in order from an object side: a first lens group having negative refractive power; a second lens group having positive refractive power; and a third lens group having positive refractive power; the front principal point of the second lens group being located to the object side of the most object side surface of the second lens group; the distance between the first lens group and the second lens group being decreased and the distance between the second lens group and the third lens group being increased when the state of lens group positions is changed from a wide-angle end state to a telephoto end state; and the following conditional expressions being satisfied: 0<H1/fw<0.6 1.0<H2/fw<2.2 1.6<f3/f2<3.0 where H1 denotes the distance between the front principal point of the second lens group and the most object side surface of the second lens group; H2 denotes the distance between the rear principal point of the second lens group and the most image side surface of the second lens group; fw denotes the focal length of the zoom lens system in the wide-angle end state; f2 denotes the focal length of the second lens group; and f3 denotes the focal length of the third lens group.
 2. The zoom lens system according to claim 1; wherein the first lens group and the second lens group are moved and the third lens group is fixed when the state of lens group positions is changed from the wide-angle end state to the telephoto end state, and the following conditional expression is satisfied: 6<M/fw<3.0 where M denotes the moving distance of the second lens group along the optical axis between the position locating the most object side and that locating the most image side upon zooming.
 3. The zoom lens system according to claim 2, wherein the second lens group is composed of, in order from the object side; a first lens unit having positive refractive power; a second lens unit having negative refractive power; a third lens unit having negative refractive power; and a fourth lens unit having positive refractive power; wherein the following conditional expressions are satisfied; 0.6<f21/f2<1.0 0.7<|f22|f2<2.0 (f22<0) 2.0<|f23|/f2<10.0 (f23<0) 1.2<f24/f2<3.0 where f21 denotes the focal length of the first lens unit; f22 denotes the focal length of the second lens unit; f23 denotes the focal length of the third lens unit; f24 denotes the focal length of the fourth lens unit.
 4. The zoom lens system according to claim ³, wherein the first lens unit includes a cemented lens constructed by, in order from the object side, a double convex positive lens having an aspherical surface facing to the object side cemented with a negative meniscus lens.
 5. The zoom lens system according to claim 3, wherein the second lens unit includes a cemented lens constructed by, in order from the object side, a double convex positive lens cemented with a double concave negative lens.
 6. The zoom lens system according to claim 3, wherein the third lens unit includes a negative meniscus lens having a convex surface facing to the object side.
 7. The zoom lens system according to claim 3, wherein the fourth lens unit includes a positive meniscus lens having a convex surface facing to the object side.
 8. The zoom lens system according to claim 3, wherein the first lens group is composed of, in order from the object side; a negative meniscus lens having a convex surface facing to the object side; a double concave negative lens having a stronger concave surface facing to the image side; and a positive meniscus lens having a convex surface facing to the object side.
 9. The zoom lens system according to claim 3, wherein the third lens group is composed of a double convex positive lens having at least one aspherical surface. 